1. CIDMA – Center for Research and Development in Mathematics and Applications, Department of Mathematics, University of Aveiro, 3810-193 Aveiro, Portugal
2. CMUC – Center for Mathematics of the University of Coimbra, Department of Mathematics, University of Coimbra, Apartado 3008, EC Santa Cruz, 3001-501 Coimbra, Portugal
3. Dept. of Math., VNU University of Education, Viet Nam National University, 144 Xuan Thuy str., Cau Giay dist., Ha Noi, Vietnam